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Uncoupling Heart Cell Specification and Migration in the Simple Research article 4811 Uncoupling heart cell specification and migration in the simple chordate Ciona intestinalis Brad Davidson*, Weiyang Shi and Michael Levine Department of Molecular and Cellular Biology, Division of Genetics and Development, University of California, Berkeley, CA 94720, USA *Author for correspondence (e-mail: [email protected]) Accepted 22 August 2005 Development 132, 4811-4818 Published by The Company of Biologists 2005 doi:10.1242/dev.02051 Summary The bHLH transcription factor Mesp has an essential but resolution visualization of heart cells as they migrate and ambiguous role in early chordate heart development. Here, divide. The enhancer was also used to drive targeted we employ the genetic and morphological simplicity of expression of an activator form of Mesp, which induces the basal chordate Ciona intestinalis to elucidate Mesp heart formation without migration. We discuss the regulation and function. Characterization of a minimal implications of Tbx6-Mesp interactions for the evolution of cardiac enhancer for the Ciona Mesp gene demonstrated cardiac mesoderm in invertebrates and vertebrates. direct activation by the T-box transcription factor Tbx6c. The Mesp enhancer was fused to GFP, permitting high- Key words: Cardiac specification, Migration, Chordate, Mesp Introduction (Haraguchi et al., 2001). Furthermore, it remains to be The Ciona heart field can be traced to a single pair of cells in determined whether Mesp functions primarily as a migration 110-cell embryos, the B7.5 blastomeres (Davidson and Levine, factor, as inferred from vertebrate analyses, or as a 2003; Hirano and Nishida, 1997). Mesp is the only regulatory specification factor, as proposed in the Ciona study. Development gene expressed exclusively in these cells (Imai et al., 2004; Here, we present evidence that Ciona Mesp is directly Satou et al., 2004). During gastrulation, the B7.5 blastomeres activated by the T-box transcription factor Tbx6c. There are divide into two distinct lineages. The rostral daughters (trunk three Tbx6 paralogs in Ciona, Tbx6a, Tbx6b and Tbx6c. Tbx6b ventral cells or TVCs) migrate to form the heart, while the and Tbx6c are activated by the maternal muscle determinant caudal daughters remain in the tail and differentiate into Macho 1, and initiate muscle gene expression (Yagi et al., anterior tail muscle cells (Fig. 1A). Homology to vertebrate 2005). While Tbx6b has a predominant role in muscle heart cells becomes evident after neurulation, when TVCs specification, Tbx6c independently regulates gene expression express orthologs of the core cardiac regulatory genes Nkx2.5, in the anterior tail muscle lineage (Yagi et al., 2005). Hand (Davidson and Levine, 2003; Satou et al., 2004) and The Mesp enhancer was used to selectively express an Gata4 (B.D., unpublished). The TVCs then migrate anteriorly activator form of Mesp in the early heart field. Heart cell and ventrally to fuse along the ventral midline in a manner migration is inhibited, but beating heart tissue nonetheless reminiscent of vertebrate heart cell migration (Davidson and differentiates at an ectopic location in the anterior tail. These Levine, 2003). After metamorphosis, the Ciona heart rudiment results demonstrate that heart specification and migration can differentiates into a contractile linear tube, which expresses be uncoupled, and implicate Mesp as a crucial cardiac orthologs of vertebrate heart structural genes including a determinant. cardiac-specific splice variant of Troponin I (MacLean et al., 1997). Mesp is expressed in the emerging heart field of mouse Materials and methods embryos prior to expression of the core cardiac regulatory Ascidians: collection, handling and experimental genes (Saga et al., 2000). There are two Mesp paralogs, and techniques chimeric cells lacking both (Mesp1 and Mesp2) display cell Ciona adults were collected from Half Moon Bay, Oyster Point and autonomous defects in heart formation (Kitajima et al., 2000). San Diego (M-Rep, CA). No significant experimental discrepancies Ciona contains a single Mesp gene. Morpholino-based were observed among animals from different sources. Rearing, fertilization, dechorionation, in situ hybridization, electroporation and suppression of Mesp function in Ciona savignyi causes a block lacZ staining were conducted as described previously (Corbo et al., in heart cell migration and specification, leading to the 1997). For double in situ hybridization, two techniques were used (the formation of supernumerary tail muscle cells (Satou et al., second protocol is a modified version of the TSA Plus Fluorescence 2004). Despite the central importance of Mesp function in early Systems protocol, Perkin Elmer, USA). (1) lacZ was stained with Fast chordate heart development, the factors that direct Mesp Red (Davidson and Levine, 2003) using a fluorescein-labeled probe, expression in the emerging heart field have not been defined while Tbx6c was hybridized to a digoxigenin-labeled probe and 4812 Development 132 (21) Research article stained using AP/NBT/BCIP as described previously (Corbo et al., using the primers VP16f (aaaaCtaGtGCaCCaCCGACCG) and 1997). (2) Tbx6c or Tbx6b were stained with Fast Red using a DIG- VP16b (aaaGAATTCCCTACCCACCGTACTCGTCAATTCC). This labelled probe, the first antibody was then stripped by incubation for fragment was then sub-cloned onto the 3Ј end of the bHLH domains 10 minutes in 100 mM glycine-HCl (pH 2.2), 0.1% Tween-20 and by using PCR-generated Spe1 and EcoR1 sites. then taken through four short rinses in PBT. The second antibody (anti-FITC-POD, Roche, USA) was subsequently applied at a 1:1500 Gel shift assays dilution overnight at 4°C. Embryos were then quenched for 20 Binding assays were conducted as described previously (Fujiwara et minutes in 0.3-3% hydrogen peroxide (in PBT), rinsed twice in PBT al., 1998). Labeled and competitor DNAs were prepared by annealing and then rinsed three times for 5 minutes in TNT buffer [0.1 M TRIS- the following oligonucleotides with their complementary fragments HCl (pH 7.5), 0.15 M NaCl, 0.05% Tween-20]. Finally, the embryos (bold underlined bases indicate putative T-box binding sites): were incubated in 300 ␮l freshly diluted (1:50) 1ϫ Plus Fluorescein Ci-B, gatcCTTAAAGGCGATAATGACTT Tyramide Stock Solution for 15 minutes and mounted in Prolong Gold Ci-C, gatcTCATGCGGCGATAAACGAAC antifade reagent (Invitrogen, USA). Ci-D, gatcACTAATTAGACACCTCCTAC Ci-B Mut-1, gatcCTTAAAGtCaATAATGACTT. Construction of transgenic DNAs The GST-Tbx6c fusion protein was expressed using a partial Tbx6c Genomic DNA was isolated from the pooled sperm of 3-4 adults, using cDNA (containing the full T-box DNA-binding domain) obtained the PureGene DNA Isolation kit (Gentra Systems), and used as a from the Ciona intestinalis Gene Collection Release1 (Satou et al., template for PCR-based isolation of the required genomic fragments. 2002) (CiGC43g03). This coding region was fused into the pGEX- These fragments were then cloned into either the pCES vector (Harafuji 5x-1 expression vector and purified from bacterial extracts using et al., 2002) or modified versions of this vector, as described below. glutathione agarose beads. Mesp reporter constructs Confocal microscopy The 5Ј flanking DNAs from Ci-Mesp and Cs-Mesp were initially Transgenic embryos with GFP-expressing cells were fixed for 1 hour isolated using the following primers [numbers indicate the base-pair in 0.3% formaldehyde in seawater, mounted in Vectashield mounting (bp) distance 5Ј of the EST predicted transcript]: medium (Vector labs, CA) and stored at –20°C. Confocal images were Ci-Mespf1916, gcgcTCTAGACGGTTCAACGTGACGTCCCAT- obtained on a Leica TCS SL1 laser scanning confocal microscope. GC; Images were processed using the BitPlane Imaris 3.3 software Ci-MespNatb, aaaGCGGCCGCCATAATACAAGTTTCAAATC- package. AACCTG; Cs-Mespf1123, gcgcTCTAGATCTGAATGAGCAG; and Cs-MespNatb, aaaGCGGCCGCCATGAATACGTTTCCAGG. Results The use of lowercase letters indicates padding on the primer that is Tbx6-binding sites are essential elements of the not incorporated into the construct. Mesp minimal enhancers These fragments were fused in-frame with lacZ in the pCES vector using the PCR-generated Xba1 and Not1 sites, replacing the To investigate how Mesp expression is restricted to the Ci-forkhead minimal promoter. These constructs were then used as emerging heart field, we isolated minimal enhancers in the 5Ј Development templates for further 5Ј deletions using the appropriate forward flanking regions of the Ciona intestinalis and Ciona savigyni primers. Mesp reporter constructs containing ~200 bp or less 5Ј Mesp genes (Ci-Mesp and Cs-Mesp). A 110-bp region of the DNA began to drive ectopic expression of lacZ in the tail muscles Ci-Mesp 5Ј flanking sequence and a 105-bp region 5Ј of Cs- and sometimes in the notochord. However, it was determined that Mesp are sufficient to drive reporter gene expression in the this ectopic expression was due to a vector artefact that was B7.5 lineage of electroporated embryos (Fig. 1B,C,F; Ci-110, eliminated by removing an ~320 bp fragment of the vector in Cs-105). Short 3- to 13-bp distal deletions eliminate the between the Xba1 site and an EcoO1091 site upstream of the polylinker. In all subsequent constructs this area of the vector was activity of both enhancers (Fig. 1D-F; Ci-107, Cs-92). removed. Single nucleotide mutations were generated by PCR Comparison of these distal sequences identified a putative amplification using primers with appropriately altered sequences. motif matching the consensus-binding
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